The present invention generally relates to electrochromic (EC) materials that exhibit different colors as a function of an applied voltage, and more specifically, to apparatus utilizing specific organic polymer based EC materials, and methods of producing the specific organic polymer based EC materials.
Electrochromic (EC) materials are a subset of the family of chromogenic materials, which includes photochromic materials, and thermochromic materials. These are materials that change their tinting level or opacity when exposed to light (photochromic), heat (thermochromic) or electricity (electrochromic). Chromogenic materials have attracted widespread interest in applications relating to the transmission of light.
An early application for chromogenic materials was in sunglasses or prescription eyeglasses that darken when exposed to the sun. Such photochromic materials were first developed by Coming in the late 1960s. Since that time, it has been recognized that chromogenic materials could potentially be used to produce window glass that can vary the amount of light transmitted, although the use of such materials is clearly not limited to that prospective application. Indeed, EC technology is already employed in the displays of digital watches.
With respect to window glass, EC materials are exciting because they require relatively little power to produce a change in their tinting level or opacity. EC windows have been suggested for use in controlling the amount of daylight and solar heat gain through the windows of buildings and vehicles. Early research indicates that EC window technology can save substantial amounts of energy in buildings, and EC glazings may eventually replace traditional solar control technology such as tints, reflective films, and shading devices (e.g., awnings). Because of their ability to control lighting levels and solar heat gain, EC windows have the potential of reducing the annual U.S. energy consumption by several quadrillion (1015) BTUs, or quads, which is a substantial decrease relative to current consumption rates.
Several different distinct types of EC materials are known. The primary three types are inorganic thin films, organic polymer films, and organic solutions. For many applications, the use of a liquid material is inconvenient, and as a result, inorganic thin films and organic polymer films appear to be more industrially applicable.
To make an EC device that exhibits different opacities in response to a voltage, a multilayer assembly is required. In general, the two outside layers of the assembly are transparent electronic conductors. Within the outside layers is a counter-electrode layer and an EC layer, between which is disposed an ion conductor layer. When a low voltage is applied across the outer conductors, ions moving from the counter-electrode to the EC layer cause the assembly to change color. Reversing the voltage moves ions from the EC layer back to the counter-electrode layer, restoring the device to its previous state. Of course, all of the layers are preferably transparent to visible light. Both inorganic and organic ion conductive layers are known.
In order to be useful in a window application, or in a display application, EC materials must exhibit long-term stability, rapid redox switching, and exhibit large changes in opacity with changes of state. For inorganic thin film based EC devices, the EC layer is typically tungsten oxide (WO3). U.S. Pat. Nos. 5,598,293, 6,005,705, and 6,136,161 each describe an inorganic thin film EC device based on a tungsten oxide EC layer. Other inorganic EC materials, such as molybdenum oxide, are also known. While many inorganic materials have been used as EC materials, difficulties in processing and slow response time associated with many inorganic EC materials have created the need for different types of EC materials.
Conjugated, redox-active polymers represent one different type of EC material. These polymers (cathodic or anodic polymers) are inherently electrochromic and can be switched electrochemically or chemically between different color states. A family of redox-active copolymers are described in U.S. Pat. No. 5,883,220. Another family of nitrogen based hetrocyclic organic EC materials is described in U.S. Pat. No. 6,197,923. Research into still other types of organic film EC materials continues, in hopes of identifying or developing EC materials that will be useful in EC windows.
While EC windows, or smart windows as they are sometimes called, are expected to represent a significant commercial application of EC technology, one additional potential use of an EC is in producing displays, sometimes referred to smart displays, or digital windows (DWs). One promising application for DW systems relates to deoxyribonucleic acid (DNA) chip reading. For more efficient DNA chip reading/writing technology, it would be desirable to replace expensive custom photomasks in the photosynthesizing of oligonucleotides in DNA array fabrication. There are several reasons why it would be desirable to develop a new method applicable to this technology for use in oligonucleotide chip manufacturing. Specifically, oligonucleotide chips have become increasingly important, as more genomes of organisms are sequenced. Accordingly, there is a need to develop a low cost, easy to use, high-density DNA arranger and system for reading unknown DNAs, based on surface plasmon resonance, with higher lateral resolution that is provided in current systems.
A suitable system for such an application should employ a switchable window that is readily changed from transparent to nontransparent (e.g., to dark blue) by varying an electric potential polarity (anodic EC polymer sides have a negative polarity and a positive polarity, respectively). These switchable window laminate materials should be convertible to a digital (pixel) array having a size typically ranging from about submicron to about 50 microns, and each array unit should be independently controlled to change from a transparent to a nontransparent state. By combining this functionality with an surface plasmon resonance (SPR) system serving as a real time analyzer of unknown molecules, including DNA sequences and characterizations of unknown molecules and in vivo and in vitro cell-cell interactions. Such a high resolution SPR system should then be useful for analyzing unknown molecules and DNA sequences on a real-time basis, at faster speed than is currently possible, by scanning through one group of molecules after another, i.e., by opening the corresponding digital window (DW) unit. With such a system, the speed with which unknown molecules (and DNA and RNA sequences) can be analyzed will be much enhanced, compared with conventional prior art techniques.
An additional exciting application of EC technology relates to the use of EC devices for display technologies, beyond the somewhat limited application of monochromatic displays now used in digital watches. EC devices that can controllably transition between more than two color states offer the potential of flat panel multicolor displays, using the digital pixel array noted above.
A first aspect of the present invention is directed to a method for synthesizing EC polymers and counter-electrodes having properties that can be beneficially employed in EC polymer devices. A second aspect of the present invention is directed to specific configurations of EC polymer based devices, while a third aspect is directed to specific applications of EC polymer devices. With respect to synthesizing EC polymers and counter-electrodes, two embodiments of the method for synthesizing EC polymers is disclosed, as well as two embodiments for fabricating counter-electrodes for use in EC devices.
The first synthesis method is directed toward the production of poly[3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine], also known as PProDOT-Me2. Preferably, equivalent molar amounts of 3,4-dimethoxythiophene and 2,2-dimethyl-1,3-propanediol are dissolved in toluene and heated in the presence of p-toluenesulfonic acid monohydrate (at a concentration of 1.5 mol % of 3,4-dimethoxythiophone) and left for 10-20 hours at a temperature of 110xc2x0 C. Those of ordinary skill in the art will recognize that the specified temperature is the boiling point of toluene. The toluene is heated to boiling, toluene vapors are collected and condensed, and then returned to the original solution (i.e. the solution of 3,4-dimethoxythiophene, 2,2-dimethyl-1,3-propanediol, toluene and p-toluenesulfonic acid monohydrate). This process is referred to in the chemical arts as refluxing.
A methanol byproduct is produced during the synthesis, and that methanol byproduct significantly reduces the rate of the reaction. Preferably, the synthesis includes the step of removing the methanol byproduct by absorbing it with calcium chloride. This can be achieved by treating the condensed toluene vapors with calcium chloride before returning the condensed toluene vapors to the original boiling solution. This will remove the methanol byproduct from the condensed toluene vapors. As those of ordinary skill in the art will recognize, such a xe2x80x9csalting outxe2x80x9d process is sometimes employed in organic synthesis to remove undesirable reactants.
A second organic polymer expected to be useful in EC devices is poly[3,6-bis(2-(3,4ethylenedioxythiophene))-N-methylcarbazole, also known as PBEDOT-NMeCz. This synthesis is somewhat more involved, requiring the formation of two intermediate compounds from readily available reagents. The intermediate compounds are then reacted in the presence of catalyst to obtain the desired product. To obtain a first intermediate compound, poly(3,4-ethylenedioxythiophene) (EDOT) is treated with n-butyl lithium in a solution of tetrahydrofuran (THF) at xe2x88x9278xc2x0 C. for one hour. The resulting intermediate compound, a Grignard reagent, is then treated with magnesium bromide diethyl etherate. That brominated intermediate product is temporarily stored in the THF solvent.
The next intermediate compound is obtained by combining dibromocarbazole (C12H6Br2NH) with lithium hydride (LiH) in dimethyl foramide (DMF), maintained at less than 10xc2x0 C. for an hour. That intermediate compound is methylated (preferably using methyl iodine, MeI or CH3I), and the temperature is raised to 50xc2x0 C. over a two hour period, yielding a methylated dibromocarbazole (C12H6Br2NCH3) intermediate product. This intermediate product is preferably purified by washing with water and ether and dried over sodium sulfate.
The two intermediate products are combined in the presence of a nickel catalyst, resulting in the EDOT rings being affixed to the derivatized dibromocarbazole. The reaction is facilitated by maintaining the mixture of the two intermediate products at 50xc2x0 C. over a twelve hour period, to yield BEDOT-NMeCz.
A first embodiment of a counter-electrode useful for EC devices can be produced by placing a thin layer of gold on a glass substrate. Preferably, the thickness of the substrate is on the order of 0.7 mm, with the gold layer being no thicker, and preferably, substantially thinner. A layer of titanium-tungsten (TiW) may be added to the glass substrate first to enhance the gold bond to the substrate. Preferably, less than 25 percent of the substrate surface is covered with the gold layer, which is deposited in a pattern that enhances conductivity across most of the surface area of the counter-electrode. Preferably the pattern includes continuous lines, such as may be achieved in a grid pattern.
A second embodiment of a counter-electrode useful for EC devices can be produced by replacing the thin layer of gold with a thin layer of highly conductive carbon, such as graphite. The TiW layer is then not required. It is preferred to include an indium tin oxide layer between the glass and the graphite.
In regard to laminated EC devices made up of at least one EC polymer, one embodiment includes both a cathodic EC polymer layer and an anodic EC polymer layer. A different embodiment utilizes a cathodic EC polymer layer and a counter-electrode layer. The embodiment utilizing two EC polymer layers includes transparent electrodes as both top and bottom layers. Indium tin oxide coated glass comprises a preferred transparent electrode. Under the top transparent electrode is disposed the cathodic EC polymer layer. A preferred cathodic polymer is PProDOT-Me2. Adjacent to the cathodic polymer layer is disposed a solid electrolyte layer. A preferred solid electrolyte comprises a gel electrolyte, having a polymer matrix, a solvent carrier, and an ion source. Lithium perchlorate (LiClO4) is a preferred ion source. One preferred gel electrolyte is polyvinyl chloride (PVC) based and another preferred electrolyte is polymethyl metracrylate (PMMA) based; both contain LiClO4. The next layer of the device comprises the anodic polymer layer, preferably comprising PBEDOT-NMeCz. A final layer comprises the transparent electrode layer noted above. In the oxidized state, when no voltage (or a positive voltage) is applied, both polymer layers are substantially clear and have a high transmittance. It should be noted however, that the PBEDOT-NMeCz never achieves a completely colorless state. When a negative voltage is applied, each EC polymer layer undergoes a reduction and changes in color from nearly transparent to dark blue. The PProDOT-Me2 layer attains a darker tint, and is thus more opaque. The color change of the device is very rapid (about 0.5xcx9c1 s) and repeatable (more than 10,000 times).
The embodiment of the EC device that has only a single EC polymer layer also includes a transparent electrode as a top layer. Again, indium tin oxide coated glass comprises a preferred transparent electrode. Examples of transparent electrodes are ITO and doped zinc oxide films deposited on transparent substrates, such as glass or plastics. After transparent electrode layer is the cathodic EC polymer layer, preferably PProDOT-Me2, followed by a solid electrolyte layer of the type described above. Adjacent to and following the solid electrolyte layer is a counter-electrode layer. A preferred counter-electrode layer includes a conductive coating applied to a transparent substrate (such as glass or plastic). Preferably, the conductive coating does not reduce transmittance through the counter-electrode layer by more than 25 percent. Preferred conductive coatings are gold and graphite, deposited in a pattern that substantially covers the substrate. A preferred pattern includes continuous lines, such as found in a grid. In the oxidized state, when no voltage negative (or a positive voltage) is applied, the PProDOT-Me2 polymer layers is substantially clear, with very little tint. When a negative voltage is applied, the counter-electrode enhances the speed with which the PProDOT-Me2 polymer layers undergoes a reduction and changes in color from nearly transparent to dark blue. The color change of the device is very rapid (about 0.5xcx9c1 s) and repeatable (more than 10,000 times).
A first preferred application specific embodiment comprises a smart window that is able to change state from substantially transparent when no voltage (or a positive voltage) is applied, to substantially opaque when a negative voltage is applied. A first embodiment of the smart window is based on a dual polymer EC device, which includes a PProDOT-Me2 cathodic polymer layer, a solid electrolyte layer, and a PBEDOT-NMeCz anodic polymer layer, as described above. A second embodiment of the smart window is based on a single polymer EC device, utilizing a PProDOT-Me2 cathodic polymer layer, a solid electrolyte layer, and a counter-electrode layer, also substantially as described above.
Still another application specific embodiment is directed to a DW for DNA chip and unknown molecules reading technology based on SPR imaging with high lateral resolution. Currently, DNA chip reading/writing technology requires expensive custom photomasks used in the photosynthesizing of oligonucleotides in DNA array fabrication. In this embodiment of the present invention, a DW including a plurality of individually addressable pixels arranged in a grid format is employed in the place of the conventional photomask. A voltage can be applied to each pixel individually, enabling selective masking to be achieved.